ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-8-697-2008Turbulent vertical diffusivity in the sub-tropical stratospherePissoI.1LegrasB.11Laboratoire de Météorologie Dynamique UMR 8539, Paris, France1202200883697707This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.This article is available from http://www.atmos-chem-phys.net/8/697/2008/acp-8-697-2008.htmlThe full text article is available as a PDF file from http://www.atmos-chem-phys.net/8/697/2008/acp-8-697-2008.pdf

Vertical (cross-isentropic) mixing is produced by small-scale
turbulent processes which are still poorly understood and
paramaterized in numerical models. In this work we provide estimates
of local equivalent diffusion in the lower stratosphere by comparing
balloon borne high-resolution measurements of chemical
tracers with reconstructed mixing ratio from large ensembles of
random Lagrangian backward trajectories using European Centre for
Medium-range Weather Forecasts analysed winds
and a chemistry-transport model (REPROBUS).
We focus on a case study in subtropical
latitudes using data from HIBISCUS campaign.
An upper bound on the vertical diffusivity is found in this case study
to be of the order
of 0.5 m<sup>2</sup> s<sup>&minus;1</sup> in the subtropical region, which is larger than the
estimates at higher latitudes.
The relation between diffusion and dispersion is studied
by estimating Lyapunov exponents and studying their variation
according to the presence of active dynamical structures.